1. Field of the Invention
This invention in general relates to voltage regulators and devices for monitoring the status of voltage regulators and, more particularly, to voltage regulators that are coupled to a line receiving alternating current ("AC") power and devices for monitoring such voltage regulators.
2. Background of the Invention
To protect equipment receiving energy from damage caused by abnormal energy transients appearing on alternating current ("AC") power feed lines, e.g., excess energy caused by a lightening strike, it is common to include in the lines feeding energy to such equipment (or to a distribution facility in a building) one or more transient protection devices. Such devices typically react to abnormal energy transients to protect the equipment by either: (i) creating an open circuit, thus isolating the equipment to be protected; (ii) diverting excess energy to a energy dissipating device; or (iii) clamping the voltage applied to the equipment to an acceptable voltage level, thus controlling the energy applied to the equipment to an acceptable level. Those transient protection devices that clamp the voltage to an acceptable level are commonly known as voltage regulators. In general, a voltage regulator is a two-terminal device that is rendered conductive when the voltage across its terminals exceeds a threshold value and that, when conductive, tends to maintain a voltage across its terminals that is close to the threshold voltage.
Because of the nature of transients, transient protection devices in general and voltage regulators in particular may be required to handle a tremendous amount of energy over a very brief period of time. For example, if the excess energy is generated by a very large energy source (e.g., a large motor in a factory) then the transient protection device, e.g., voltage regulator, must dissipate or otherwise handle the energy remaining after the energy on the protected line is clamped to the threshold level. If the energy supplied to the transient protection device is more than the device can physically dissipate over the allowable time interval, a possibility exists that the transient protection unit might not be able to contain the excess energy, and in such cases, the transient protection device might fail. Such failures may result in the protection device failing "open" wherein the device forms a permanent open circuit or, more likely, the protection device may fail "short" resulting in an a permanent short circuit. For purposes of this disclosure, an open circuit includes both completely open circuits (i.e., essentially infinite resistance between the open parts of the circuit) and circuits wherein the resistance between the open parts of the circuit is so high that no significant current flows between the points. In the same sense, a closed circuit includes circuits wherein the resistance between the points on the circuits is low enough that significant current flows between the points.
Because of the potential importance of transient protection devices to the protection of equipment, it is often important to know whether a particular transient protection device is operable or has failed. In the past, transient protection device monitors have been used for certain classes of transient protection devices, but such monitors are often expensive, require a large number of parts, or require the addition of undesirable elements. For example, past monitors used to determine the operational status of voltage regulators typically included a fuse placed in series with the regulator. The voltage across the fuse would be monitored and this voltage would provide an indication of the current state of the monitored device. Moreover, such fused-based circuits cannot determine the integrity of a voltage regulating device in and of itself because the fused-based systems monitor the only the fuse's integrity and, thus, cannot detect if a voltage regulating device is present or operable. Accordingly, such systems cannot detect voltage regulator failures if the failure is cause by an open voltage regulator.
The above-described, and other limitations, of earlier devices for monitoring the integrity of voltage regulator devices are overcome by the present invention.